There is a need to increase the use of light weight metal alloys in the manufacture of automotive vehicles. For example, aluminum alloy sheet workpieces are formed in elevated temperature forming processes into inner and or outer vehicle closure panels. And there is a desire to use magnesium alloys in like hot forming operations.
Certain aluminum alloys may be prepared in sheet form to display a high level of formability. Some such aluminum sheet alloys (e.g., AA5083) are heated to temperatures of the order of 450° C. and hot blow formed into and stretched against shaped forming surfaces of suitable tools to form vehicle tailgate panels, door panels and the like. While the hot aluminum material is quite formable, it tends to adhere to hot tool surfaces, requiring special tool alloys, special surface coatings for the tools, and frequent cleaning or repair of the forming surfaces. Any adhesion of an aluminum workpiece to its forming tools is of particular concern since sticking of workpiece material on the forming tool directly influences the final quality of each formed part.
Other aluminum alloys (AA5xxx and 6xxx) or magnesium alloys are heated to hot stamping/forming temperatures (e.g., about 300° C. and higher) and formed between suitable forming dies. Again, the hot aluminum sheet material tends to adhere to tool surfaces to the detriment of the formed parts and uninterrupted forming operations.
It is a purpose of this invention to provide sheet metal forming tools made of readily available hard tool steel compositions with surfaces displaying reduced friction and less adhesion when the tools engage and deform light metal alloy workpieces.